1. Field of the invention
The present invention relates to a heat fixable toner used for obtaining a fixed toner image by forming a toner image using an image forming process such as electrophotography, electrostatic printing or magnetic recording, and heat-fixing the formed toner image on a recording medium. It also relates to a heat fixing method making use of such a toner.
2. Related Background Art
As a method of fixing a visible image of toner onto a recording medium, a heat-roll fixing system is widely used, in which a recording medium retaining thereon a toner visible image which has not been fixed is heated while it is held and carried between a heat roller maintained at a given temperature and a pressure roller having an elastic layer and coming into pressure contact with the heat roller.
A belt fixing system is known, as disclosed in U.S. Pat. No. 3,578,797.
The above conventional heat-roll fixing that has been hitherto widely used, however, has the following disadvantages:
(1) A time during which an image-forming operation is prohibited, i.e., what is called a waiting time, is required until the heat roller reaches the given temperature.
(2) The heat roller must be maintained at an optimum temperature in order to prevent poor fixing caused by the variations of the heat-roller temperature that may occur when the recording medium is passed or because of other external factors, and also to prevent the transfer of toner to the heat roller, i.e., what is called the offset phenomenon. This makes it necessary to make large the heat capacity of the heat roller or a heater element, which requires a large electric power.
(3) After the recording medium has been passed over the heat roller, the recording medium and the toner on the recording medium are slowly cooled because of a high temperature of the heat roller. This results in a high adhesion of the toner. Thus, conjointly with the curvature of the roller also, there may often occur offset, or paper jam caused by the rolling-up of the recording medium to the roller.
(4) A protective member must be provided on account of safety since there is a possibility of directly touching the high-temperature heat roller.
The above problems (1) and (2) in the heat-roll fixing are not fundamentally solved also in the belt fixing system disclosed in U.S. Pat. No. 3,578,797.
Japanese Patent Application Laid-open No. 63-313182, previously proposed by the present applicant, provides an image forming apparatus with a shorter waiting time and a low power consumption, comprising a fixing unit in which a toner visible image is heated through a movable heat-resistant sheet by means of a heating element having a low heat capacity, pulsewise generating heat by electrification, and is thus fixed to a recording medium. Japanese Patent Application Laid-open No. 1-187582, previously proposed by the present applicant, provides a fixing unit for heat-fixing a toner visible image to a recording medium through a heat-resistant sheet, wherein said heat-resistant sheet comprises a heat-resistant layer and a release layer or a low-resistant layer, thereby effectively preventing the offset phenomenon.
In addition to the factors in the above fixing apparatus, however, achievement of both the fixing performance of an excellent toner visible image to a recording medium and the prevention of offset and simultaneous realization of a fixing method with a shorter waiting time and a low power consumption greatly depend on the properties of a toner.
As materials suited for a low-temperature fixable toner, polyester resins with a low molecular weight have attracted notice. An attempt to use a polyester resin as a binder for a toner is seen in U.S. Pat. Nos. 3,590,000 and 3,681,106, and Japanese Pat. Publications No. 46-12680 and No. 52-25420. Since, however, melt viscosity of the resin is lowered when the molecular weight is made smaller with the aim of lower-temperature fixing, the temperature control of a fixing device is so adjusted as to yield a temperature at which a toner can be sufficiently fixed. This may cause the offset phenomenon in which the toner is melt-adhered not only to paper, but also onto a heater element.
Concerning the prevention of offset in respect of a toner comprising a polyester resin as a main binder, Japanese Patent Publication No. 52-25420 proposes a method in which a polymer is made non-linear by mixture of a polyol having three or more hydroxyl groups or a poly acid to improve its viscoelasticity so that the offset resistance at the time of fixing can be improved. However, making the polymer non-linear until the toner can have a sufficient offset resistance may result in a rise of its fixing point, and hence this method is not suitable for a toner intended to be fixed at a low temperature.
As proposed in Japanese Patent Application Laid-open No. 59-9669, it has been attempted to mix a polyvalent metal compound so as to give a cross-linked structure by virtue of metal ions, so that the polymer chains can mutually act to change melt viscoelasticity, thus preventing the offset. The polyvalent metal compound, however, has so low a specific resistance compared to resins that the static chargeability of a toner may be lowered. For this reason, if the prevention of offset relies only on the cross-linking of the polyvalent metal compound, the compound is necessarily added in a larger amount, which tends to lower the development performance of the toner. In this way, the low-temperature fixability and the offset resistance are performances that conflict with each other, and it is very difficult to combine these. As a means for solving this problem, Japanese Patent Applications Laid-open No. 60-67958 and No. 64-15755 propose a method in which a low-molecular weight polyester for achieving the low-temperature fixing and a high-molecular weight polyester for achieving the offset resistance are blended. This has made it possible to achieve the low-temperature fixing while keeping the offset resistance when compared with conventional polyesters.
However, in an attempt to achieve a better low-temperature fixing, the low-molecular weight polyester to be blended must be made to have a low melting point, resulting in a lowering of the offset resistance. When a polyester made to have a molecular weight high enough to well compensate for this lowering is mixed with the low-molecular weight polyester, it is difficult for the components added to a toner as exemplified by a coloring agent, to be uniformly dispersed and it has been impossible to obtain good development performance.
Japanese Patent Application Laid-open No. 60-4947 proposes a toner in which an organic metal compound is added to a mixture of a linear polyester and a non-linear polyester resin, which is then cross-linked. In this instance, however, both of the linear polyester and the non-linear polyester undergo cross-linking, and the non-linear polyester, in particular, is made high-molecular. Hence, there is room for further improvement when low-temperature fixing is desired.